THE RELATION BETWEEN LAND SUBSIDENCE, ACTIVE QUATERNARY FAULTS, AND EARTH FISSURES IN LAS VEGAS VALLEY, NEVADA

More than 1.7 m of land subsidence due to groundwater withdrawal has occurred in Las Vegas Valley, Nevada since 1963 with movement concentrated in four localized subsidence bowls. Although previous maps based on conventional geodetic measurements showed subsidence to be uniformly distributed within these bowls, our synthetic aperture radar interferometry (InSAR) studies now show that movement is strongly controlled by the active Quaternary faults that cut the sedimentary basin-fill deposits. The InSAR mapping for 1992-1999 reveals that subsidence is occurring in a series of elongated bowls that are each bounded by faults; in particular, the Northwest bowl is sharply bounded by the Eglington fault which is apparently acting as a subsidence barrier. Earth fissures are the most significant effect of subsidence in Las Vegas Valley, accounting for millions of dollars in damage since the 1980's. Our previous (1991) studies of earth fissures in the valley showed that more than 90% of mapped fissures lie within 600 m of known faults, indicating that the faults serve as preferred planes of weakness. Simulation modeling by Helm (1994) and Burbey (2002) suggested that pumping-related horizontal strain may be localized along faults and account for fissuring. One of the most extensive areas of earth fissures is located along the Eglington fault, where vertical subsidence rates have been as high as 5-6 cm/yr. Our recent InSAR results together with conventional leveling indicate that the vertical movement across the Eglington fault, however, is opposite (antithetic) to the original geological sense of displacement; at least two other faults associated with fissures show similar senses of opposite motion. This relation suggests that the kinematic motion of the reactivated faults may be generating a horizontal component of strain as the geologic footwall block subsides. We are currently studying the use of ascending and descending SAR interferograms for differentiating the vertical and horizontal components of ground movement.